1. Field of the Invention
The present invention relates to a transmission line and a flat cable that transmit a plurality of high frequency signals.
2. Description of the Related Art
Various transmission lines that transmit a high frequency signal have been devised. For example, Japanese Patent Laid-Open publication No. H04-144301 discloses a transmission line with a stripline structure. The transmission line disclosed in Japanese Patent Laid-Open publication No. H04-144301 is provided with an elongated stacked insulator that extends in a direction in which a high frequency signal is transmitted, a plurality of signal lines, a first ground conductor, and a second ground conductor. The plurality of signal lines are arrayed in parallel to the first ground conductor and the second ground conductor, alternately being shifted vertically to each other. With this configuration, the characteristic impedance of the signal lines is matched with a predetermined value, which makes it possible to arrange the plurality of signal lines with high density.
In a transmission line including the structure disclosed in Japanese Patent Laid-Open publication No. H04-144301, it is difficult to adjust the impedance of each signal line to a desired impedance. In addition, since a ground layer that overlaps in a stacking direction is shared, coupling through the ground layer is likely to occur. In other words, leakage of a signal between transmission lines due to a feedback current is likely to occur.
On the other hand, a structure in which, by arranging an interlayer connection conductor between signal lines, unwanted coupling between adjacent signal lines is significantly reduced or prevented by the interlayer connection conductor, is able to be considered.
In such a configuration in which an interlayer connection conductor is arranged between adjacent signal lines, however, the following problems may occur.
In a case of using a structure in which the pitch of the interlayer connection conductor arranged between the adjacent signal lines is made rough or a structure in which the interlayer connection conductor is eliminated, unwanted coupling between the signal lines is likely to occur. On the other hand, if the pitch of the interlayer connection conductor is made fine, the capacitance between the signal line and the interlayer connection conductor becomes larger and a desired characteristic impedance as a transmission line is difficult to be achieved.
As the width of the stacked insulator is smaller, the capacitance that is generated between the interlayer connection conductor and the signal line becomes larger and thus a desired characteristic impedance as a transmission line is difficult to be achieved.